1. Field of the Invention
This invention relates generally to a flat envelope type fluorescent character indicating tube wherein a plurality of digit indicating elements are arranged in an evacuated envelope and more specially to an improved tube having a new and improved getter support structure.
2. Description of the Prior Art
There are many types of fluorescent character indicating tubes of the character described presently available, and one is a flat envelope type fluorescent character indicating tube wherein a plurality of co-planar letters and/or numeral indicating anode segments are arranged on an insulating base plate, and the fluorescent material is disposed on each of the anode segments. The visible location of digits is controlled by the mesh control grids which are disposed in front of the anode segments.
As this type of tube is formed by sealing a flat envelope cover with the base plate mentioned above, it is possible to obtain a compact electron tube as compared to the conventional elongated circular envelope type tube.
However, this type of tube has some technical problems resulting from its compact size.
One is the distribution of the evaporated getter film.
It is well known that, in order to maintain the required degree of vacuum in an electron tube, the generally used technique is to employ a getter. The getter consists of a thin metallic film deposited on appropriate surfaces of the tube envelope. This thin metallic film is usually of barium and is commonly deposited from a getter source container which may be heated by externally induced radio frequency currents.
As a matter of fact, the flat envelope type fluorescent character indicating tube also employs a getter for absorbing the residual gases in its vacuum envelope.
But, in this type of tube, it happens that the digit indicating elements are made dirty and the insulation among the electric circuits is spoiled on account of the evaporated getter film. Also if the getter film is deposited on the surface which faces the digit elements, the light indicating the indicia would become obscure.
Another difficulty is the so called "end cooling effect" of the cathode filament.
In this type of tube, a directly heated filament is disposed in front of the mesh control grid for emitting electrons toward the phosphor segments. As the filament is usually stretched by a pair of support elements, the amount of emission of electrons from near the filament supporting point is usually reduced, as compared to the amount from other points of the filament. This reduction of electron emission is called an end cooling effect, and it influences the brightness of the digit, especially that being located near the end of the filament. To avoid this influence, it is necessary to support the filament at a location as far as possible from the end digit element. However, in doing so, it becomes difficult to obtain an envelope of a compact size.